Expose activeStreams and potentially pendingStreams for use in ConnectionPoolMetrics in MeterRegistrar

[FlattestWhite]

We use our own metrics vendor and don't use micrometer. To instrument the httpclient, we have an implementation of MeterRegistrar. There's currently no way to get access to activeStreams in ConnectionPoolMetrics. The provided micrometer metrics internally cast to an Http2Pool before calling its activeStreams method.

Motivation

To work around this, we have to keep a count of concurrent requests through a counter (increment and decrement) and report that. Would prefer to have it come through in ConnectionPoolMetrics

Desired solution

Considered alternatives

Additional context